Manufacture of silicon carbide foam from a polyurethane foam impregnated with resin containing silicon

ABSTRACT

The invention concerns a process for preparing a silicon carbide foam consisting of attacking a polyurethane foam with an alkaline solution, impregnating it, after rinsing and drying, with a suspension of silicon powder in an organic resin, heating progressively to polymerise the resin, carbonising the polyurethane foam and resin, and finally carburising the silicon contained in the resin suspension by means of the carbon originating from the carbonisation of the foam and resin. 
     The foams obtained are characterised by a high microporosity and a mesoporosity which is variable according to the carburising temperature. 
     The invention finds an application in the manufacture of catalyst carriers for exhaust chambers and filters for diesel engines.

This is a divisional of application Ser. No. 08/234,962, filed Apr. 28,1994, now U.S. Pat. No. 5,429,780.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the technical field of porous structures orfoams made from silicon carbide intended to serve as catalysts orcatalyst carriers for the chemical or petrochemical industry orcatalysts or filters for exhaust silencers.

THE PROBLEM POSED

The specific surface area of a catalyst is due to three types ofporosity: a macroporosity due to pores with a mean diameter greater than2 μm, a mesoporosity due to pores with a mean diameter of 30-350angstroms and a microporosity due to pores with a mean diameter of 5-15angstroms. A catalyst must have sufficient macroporosity to give the gasto be treated access to the micro- and especially the mesoporesresponsible for the catalytic activity proper.

For certain catalytic applications, in particular for catalysing theoxidation of exhaust gases, it is necessary to choose the geometry ofthe carrier or catalyst and to produce it in the form of monolithicpieces, ensuring that the catalyst is accessible to the gas to betreated by its macroporosity. With exhaust filters for diesel engines,the problem is substantially the same, except that the mesoporosity doesnot need to be as highly developed since the filtration involves onlyphysical phenomena and not a catalytic reaction.

The inventors posed themselves the problem of obtaining catalystcarriers made from practically pure silicon carbide foam in monolithicform and having a large open macroporosity allowing easy access to thereaction gases.

OBJECT OF THE INVENTION

The invention relates to a process for manufacturing catalyst carriersor filters made of silicon carbide and having the properties indicatedabove, from a polyurethane foam which is impregnated with a suspensionof silicon in a polymerisable organic resin and which is raised to ahigh temperature in order successively to polymerise the resin, tocarbonise the resin and polyurethane foam and finally to carburise thecarbon originating from the resin and foam.

It also relates to porous silicon carbide structures which can beapplied to the production of catalytic chambers or filters for dieselengines.

THE PRIOR ART

European patent EP-B-0313480 (Pechiney Electrometallurgie) discloses aprocess for producing silicon carbide grains with a specific surfacearea of at least 100 m² /g, intended to serve as a catalyst carrier, andconsisting of generating SiO vapours by heating a mixture of SiO₂ +Si toa temperature of between 1100° and 1400° C. at a pressure of between 0.1and 1.5 hPa and reacting these vapours on reactive carbon with aspecific surface area greater than 200 m² /g at a temperature of between1100° and 1400° C.

European patent application EP-A-0440569 (Pechiney Electrometallurgie)describes a process for obtaining porous solid bodies of refractorycarbide with a high specific surface area, characterised in that apolymeric and/or copolymerisable organic compound which is carbonisableand capable of giving a solid carbon network is mixed with a metalpowder or metalloid powder or one of its compounds which can be reducedby carbon, the mixture is shaped, the organic compound is cross-linkedor hardened, and the shaped mixture is heat-treated in order tocarbonise the said compound between 500° and 1000° C. and then to effectcarbonisation.

European patent application EP-A-0511919 (Pechiney Electrometallurgie)describes a carrier on which the catalytically active product isdeposited. The carrier has mechanical or physical properties which areof interest for the required operating conditions, but a mediocrespecific surface area. The catalytically active product, a metalliccarbide, is obtained by immersing the carrier in a suspension of areducible compound of the metal in a solution of an organic compound,carbonising this compound, reducing the metallic compound andcarburising the metal. The carbide thus obtained has a high specificsurface area. The carrier preferably consists of silicon carbideprepared by carbonising a paste containing silicon, carbon and anorganic resin.

The unpublished European patent application 92-420429 (GIE PechineyRecherche) describes a metallic carbide foam or silicon carbide foamintended to be used as a catalyst or catalyst carrier for the chemicalor petrochemical industry or for exhaust silencers, and the process formanufacturing it. The foam is in the form of a three-dimensional networkof interconnected cages, the length of the edges of which is between 50and 500 micrometers and the BET surface area of which is between 20 and100 m² /g. The manufacturing process consists of starting from a carbonfoam, increasing its specific surface area by an activation treatment bymeans of carbon dioxide, and finally bringing the foam thus activatedinto contact with a volatile compound of the metal whose carbide it isdesired to obtain.

None of these above processes describes bringing an organic suspensioncontaining silicon into contact with an organic foam precursor.

The document EP-A-0337285 describes the obtaining of a mineral foam froma polyurethane foam but, since the final pyrolysis takes place innitrogen, a surface deposit consisting of SiO₂ and/or silicon oxynitrideis obtained rather than solid silicon carbide (see EP 0337285, A2,column 8, lines 45-47).

DESCRIPTION OF THE FIGURES

The single FIG. 1 shows the distribution of the porous volume as afunction of the diameter of the pores in a sample of foam according tothe invention.

DESCRIPTION OF THE INVENTION

The process for manufacturing silicon carbide with a large specificsurface area from a polyurethane foam according to the inventionconsists of:

a) the preparation of this polyurethane foam;

b) elimination, if necessary, of any solvent by heating;

c) polymerisation of the resin by progressively increasing thetemperature to 250° C. with a rate of temperature rise of around 5°C./min;

d) simultaneous carbonisation of the polyurethane foam and resin byprogressively increasing the temperature from 250° C. to 1000° C. in aninert atmosphere;

e) carburising of the silicon contained in the resin suspension by meansof the carbon originating from the carbonisation of the foam and resinby progressively increasing the temperature from 1000° C. to atemperature T of between 1300° and 1600° C. with a rate of temperaturerise of around 3° C./min in an inert atmosphere and maintaining at thetemperature T for two hours, all the time in an inert atmosphere;

f) cooling of the silicon carbide thus obtained;

and is characterised in that the preparation a) of the polyurethane foamconsists of:

a1) impregnation of the latter with a suspension of Si in powder form inat least one oxygenated organic resin and an optional solvent, the ratioby weight of Si to the resin being between 0.58 and 1.17 and preferablybetween 0.58 and 0.8, and

a2) elimination of the excess suspension so that the ratio of the weightof resin to the weight of polyurethane is between 2 and 2.5.

Each of these successive steps is now described in detail.

a1) impregnation of the polyurethane foam with a suspension of siliconpowder in an organic resin.

a2) elimination of the excess suspension so that the ratio of the weightof resin impregnating the polyurethane to the weight of polyurethane isbetween 2.5 and 5;

The choice of the resin is dictated by two considerations: its viscosityand the percentage by weight of oxygen in the resin moleculepolymerised. The viscosity must be chosen so as to be fairly low so thatthe resin suspension can penetrate to the heart of the foam, and lessthan 3 Pa.s. This viscosity may be adjusted by diluting the resin with asolvent such as alcohol or by adding very fine carbon black. Preliminarytests, consisting of immersing cubic samples of foam with an edge of 30mm and checking whether the resin has succeeded in penetrating to theheart, may usefully be applied.

The percentage by weight of oxygen has great importance since, at thetime of carbonisation of the impregnated foam, it assists, through agradual oxidation of the carbon which is forming, in activating thiscarbon, that is to say developing its mesoporosity and increasing itsspecific surface area. It has been found that the proportion by weightof oxygen in the polymer obtained from the resin ought to be at least15% and preferably around 25%. Amongst these resins are foundpolycarbonates, polyesters and especially polymers obtained bypolycondensation of furfuryl alcohol in which the proportion of oxygenis approximately 26%, which constitute the preferred polymer.

The distribution of the diameters of the silicon grains must be centredon a mean value D50 (by volume) of less than 75 micrometers andpreferably less than 10 micrometers.

The quantity and composition of the impregnations suspension are definedby two coefficients:

a=weight of resin/weight of polyurethane=M_(r) /M_(pu)

b=weight of silicon/weight of resin=M_(Si) /M_(r)

In the following, when it is a question of the weight M_(r) of resin,the total resin plus any polymerisation catalyst must be understood. Forexample, hexamethylenetetramine is added as a catalyst to the preferredresin, which is furfuryl alcohol.

With regard to the coefficient a, it is clear that if it is too low, thecarbon structure obtained will be very porous and will therefore haveinsufficient mechanical strength; if it is too high, the macroporousstructure will be blocked by the products of carbonisation of the resinand will therefore not be preserved. It is however necessary for thequantity of resin to be sufficient to impregnate the mass ofpolyurethane as far as the heart. It may thus prove necessary to adjustthe viscosity of the resin when hot by acting on the quantity ofpolymerisation catalyst and the viscosity when cold by diluting it inalcohol or adding carbon black.

The inventors have found that this coefficient a ought to be between 2.5and 5. This limitation of the value of a is explained in this way:

The carbon originating from the carbonisation of the polyurethane andresin occupies a certain "real" or "solid" volume (the quotient of theweight of carbon M_(c) and its density d_(c)) equal to V_(c). In orderto obtain sufficient strength of the carbon foam, it is necessary forthis volume of carbon V_(c) to occupy at least 25% of the "real" or"solid" volume V_(pu) of the initial polyurethane (the quotient of theweight of polyurethane M_(pu) and its density d_(pu)). In order to keepa macroporous structure of the carbon foam, it is necessary for thisvolume of carbon V_(c) to occupy at least 100% of the "real" or "solid"volume V_(pu) of the initial polyurethane. If the ratio V_(c) /V_(pu),which is a volume yield of the carbonisation, is termed x, this shouldtherefore give:

    0.25<x<1

A simple calculation enables the weight yield R_(x) corresponding to xto be calculated: ##EQU1##

The coefficient b is derived easily from the stoichiometry of the SiCformation reaction:

    Si+C=SiC

The weight of silicon M_(Si) =(28/12)·M₃ =2.333·M_(c) (atomic weights ofSi and C=28 and 12 respectively). According to (1), M_(c)=(R/100)·M_(pu) ·(1+a)

Finally this gives:

    Msi=2.333·(R/100)·Mpu·(1+a)     (2)

and

    M.sub.Si =b·M.sub.r where b=(M.sub.Si /M.sub.pu)·(M.sub.pu /M.sub.r) b=2.333·(R/100)·(1+a)·(1/a)    (3)

In the field of the invention, x is generally around 0.5 and, thedensities of carbon and polyurethane being approximately 2 and 0.8respectively, formula (1) is written:

    R=100·(0.5·2/0.8)/(1+a)=125/(1+a)        (4)

Combining (3) and (4), the value of b=M_(Si) /M_(r) is equal to:

    b=2.333·(R/100)·(1+a)·(1/a)=2.917/a(5)

The last equation (5) states that, for a weight b.M_(r) of silicon,added to a weight M_(r) of resin during the preparation of the mixture,the ratio a of the weight of resin to the weight of polyurethane shouldnot exceed 2.917/b.

For example, for a value of b of 0.79, the maximum value of a is 3.7 andthe corresponding yield R is approximately 27%.

For the extreme cases:

x=0.25 and a=2.5, b is equal to 0.58

x=1 and a=5, b is equal to 1.17

In practice, a suspension of silicon in the resin is prepared, in theproportions defined by coefficient b. The foam is impregnated simply byimmersion in this resin, optionally under vacuum or under pressure. Acheck is made that the foam is indeed impregnated to its heart. Ingeneral the degree of impregnation, verified by weighing, is greaterthan the value of the sought-for coefficient a. The excess quantity ofresin is then eliminated by centrifuging until this coefficient a isobtained, which is checked by weighing. This centrifuging at 1000rev/min may take several hours.

In order to check, by weighing, the quantity of impregnation product inthe impregnated foam, the ratio c of the total weight (foam+impregnationproduct) to the weight of polyurethane foam is used. This ratio is equalto:

    c=(M.sub.pu +M.sub.r +M.sub.Si)/M.sub.pu ·(1+a+b.a)/M.sub.pu =1+a·(b+1)                                       (6)

or again, by combining (5) and (6):

    c.sub.max =3.917+a=3.917+2.917/b

The centrifuging will therefore be checked by successive weighings ofthe sample. This will be continued until the total weight is equal to orless than c_(max) ·M_(pu).

d) polymerisation of the resin contained in the suspension byprogressively increasing the temperature to 250° C. with a rate oftemperature rise of around 5° C./min;

e) simultaneous carbonisation of the polyurethane foam and resin byprogressively increasing the temperature from 250° C. to 1000° C. with arate of temperature rise of between 1° C./min and 10° C./min in an inertatmosphere;

Inert atmosphere means an atmosphere which is non-reactive vis-a-vis Siat high temperature; it must in particular contain less than 100 ppm ofoxygen and less than 1% nitrogen; it advantageously consists of a flowof argon of commercial purity.

It has also been remarked that the best properties of the final Sicarbide are obtained if the mean velocity of the scavenging gas duringthis step is between 0.0 and 1 cm/sec. and preferably between 0.05 and0.2 cm/sec.

f) carburising of the silicon contained in the suspension of resin bythe carbon originating from the carbonisation of the foam and resin byprogressively increasing the temperature from 1000° C. to a temperatureT of between 1300° and 1600° C. with a rate of temperature rise ofaround 3° C./min in an inert atmosphere and maintaining at thetemperature T for 2 hours, all the time in an inert atmosphere.

Verification that all the carbon available has been converted into SiCcan be effected simply by weighing the carbide foam. In fact, the weightof SiC, Msic, is related to the weight of carbon M_(c) by thestoichiometric equation:

    M.sub.SiC =M.sub.c ·(40/12)=3.333·M.sub.c

According to equation (1),

    R.sub.x (as a fraction)=M.sub.c /(M.sub.pu ·(1+a))

    M.sub.c =R.sub.x ·M.sub.pu (1+a)

    M.sub.SiC /M.sub.pu =3.333·R.sub.x ·(1+a)

If equation (4) is satisfied, this finally gives:

    M.sub.SiC /M.sub.pu =3.333·1.25=4.17              (8)

This last equation makes it possible to calculate the theoretical weightof silicon carbide which must be obtained from a given weight of initialpolyurethane.

The choice of the final temperature T is dictated by the size of themesoporosity volume which it is desired to achieve. The higher thetemperature T, the smaller this volume. Thus, for preparing catalystcarriers, a temperature T of between 1300° C. and 1400° C. is preferred,whilst for preparing diesel engine filters, a higher temperature T ispreferred, between 1400° C. and 1600° C.

The product obtained is in the first case a silicon carbide catalyst orcatalyst carrier having a large open macroporosity allowing easy accessto the reaction gases, characterised in that the BET surface area isbetween 10 and 50 m² /g, in that its bimodal porosity comprises amesoporosity centred on a pore diameter value of between 0.0275 and0.0350 μm with a standard deviation of less than 0.0160 μm and amacroporosity centred on a pore diameter value of between 100 and 150 μmwith a standard deviation of less than 50 μm, and in that the carbidecontains not more than 0.1% by weight of residual silicon.

If the initial polyurethane foam does not have open porosity or hasinsufficient open porosity for the application envisaged, it ispossible, before steps a1) and a2), to attack the polyurethane foam witha soda solution so as to obtain the desired open porosity. This attackis preferably effected in a 4% (by weight) soda solution at atemperature of approximately 65° C. for approximately 5 to 10 min.

The permeability of the carbides according to the invention was comparedwith that of a conventional catalytic support, made from cordierite,with the same shape and same dimensions. This is, in fact, acharacteristic of use which is very important with exhaust silencercatalysts, which must not cause an excessively high pressure drop in thehigh level of flow of exhaust gases.

A pressure drop per unit of length P/1 is given by the formula:

    P/1=μ·d/k.sub.1 ·S+m·d.sup.2 /k.sub.2 ·S.sup.2

where

P is the pressure drop

1 the length of the sample

S the cross section of the sample

μ the viscosity of the fluid

m the density of the fluid

d the volume flow rate of the fluid

k₁ and k₂ two coefficients characteristic of the permeability of thesample, k₁ being the so-called darcian permeability coefficient and k₂the so-called non-darcian permeability coefficient.

It should be noted that, all other things being equal, the higher k₁ andk₂, the smaller the pressure drop P, that is to say the higher thepermeability.

The following table gives the values of the coefficients k₁ and k₂ forthe conventional cordierite catalyst and for the SiC foam according tothe invention.

    ______________________________________                                                        k.sub.1 (m.sup.2)                                                                    k.sub.2 (m)                                            ______________________________________                                        Cordierite catalyst                                                                             9 × 10.sup.-9                                                                    1.4 × 10.sup.-4                              SiC foam          8 × 10.sup.-9                                                                    1.4 × 10.sup.-4                              ______________________________________                                    

It is clear from this table that the coefficients k₁ and k₂ are of thesame order of magnitude for the silicon carbide according to theinvention as for the cordierite and that their permeabilities aretherefore comparable.

The silicon carbide foam can therefore, like the cordierite, beimpregnated thereafter with an active phase based on platinum, rhodiumor palladium serving as a catalyst.

The product obtained is in the second case a silicon carbide filter witha large open macroporosity allowing easy access to the gases,characterised in that its BET surface area is less than 5 m² /g, in thatits porosity comprises a very low mesoporosity and a macroporositycentred on a pore diameter value of between 100 and 150 μm with astandard deviation of between 30 and 50 μm, and in that the carbidecontains no more than 0.1% by weight of residual silicon.

EXAMPLES Example 1

A sample of polyurethane foam with a volume of 90 cm³ and an apparentdensity of 0.03 g/cm³, is attacked by a 4% soda solution at 55° C. for 8minutes. A weight loss of 18.4% is noted. The foam, which weighed 2.7 gat the outset, therefore weighs no more than 2.20 g. A mixture of 95%(by weight) furfuryl alcohol and 5% hexamethylenetetramine is prepared,serving as a polycondensation catalyst, but the action of whichcommences only as from approximately 170° C. The mixture thereforeremains stable at ambient temperature, which allows the subsequentoperations without polymerisation of the resin. Silicon in powder formwith a mean grain diameter of 60 μm, is added in a proportion of 7.9 gof silicon for 10 g of resin. The coefficient b is therefore equal to0.79. The polyurethane foam is then immersed in the suspension obtaineduntil the impregnation reaches the heart of the foam. Centrifuging isthen carried out so that the weight of resin impregnating the sample isreduced to 5.52 g. The weight of silicon is then 4.36 g and the totalweight of the impregnated sample is 12.08 g. The coefficient a is equalto 5.52/2.20= 2.51 and the ratio c is 12.08/2.20=5.49, a value lessthan: c_(max) =3.917+2.917/b=7.61.

There is therefore a lack of impregnation, which will result in aquantity of SiC less than the theoretical quantity and the presence ofan excess of carbon in the carburised foam.

The impregnated foam is then heat-treated in order, successively: topolymerise the resin by increasing the temperature to 250° C. with arate of temperature rise of around 5° C./min, to carbonise thepolyurethane foam and resin by increasing the temperature from 250° C.to 1000° C. with a rate of temperature rise of around 1° C./min in anon-oxidising atmosphere; to carburise the silicon by means of thecarbon originating from the carbonisation of the foam and resin byincreasing the temperature from 1000° C. to 1350° C. with a rate oftemperature rise of around 3° C./min in a non-oxidising atmosphere andmaintaining for 2 hours at 1350° C., all the time in a non-oxidisingatmosphere.

After this treatment, the weight of foam obtained is 6.98 g. After afurther treatment in air at 800° C., a weight loss of 6.5% is observed,corresponding to the combustion of the excess of carbon, which reducesthe weight of SiC foam to 6.53 g, less, according to (8), than themaximum theoretical quantity of 4.17×2.20=9.174 g. The quantity ofresidual silicon is very much less than 1%.

Example 2

A sample of polyurethane foam with a volume of 43.75 cm³ and an apparentdensity of 0.07 g/cm³, is attacked by a 4% soda solution at 65° C. for 8minutes. No weight loss is noted, the used foam having an open porotisy.The sample therefore weighs 3.06 g. A mixture of 95% (by weight)furfuryl alcohol and 5% hexamethylenetetramine is prepared as inExample 1. Silicon in powder form with a mean grain diameter of 60 μm isadded to this mixture in a proportion of 7.9 g of silicon for 10 g ofresin as in Example 1 and, in order to facilitate impregnation, 0.15 gof carbon black, still for 10 g of resin. The coefficient b remainsequal to 0.79. The polyurethane foam is then immersed in the suspensionobtained until the impregnation reaches the heart of the foam.Centrifuging is then carried out so that the weight of resinimpregnating the sample is reduced to 11.74 g. The weight of silicon isthen 9.28 g and the total weight of the impregnated sample is 24.08 g.The coefficient a is equal to 11.74/3.06=3.84 and the ratio c is24.08/3.06=7.86, a value slightly greater than:

    c.sub.max =3.917+2.917/b=7.61

There is therefore a slight impregnation excess, which should cause aslight excess of silicon compensated for however by the supplementaryaddition of carbon black to the resin. In fact, this is very close tothe theoretical quantities of carbon and silicon, which will result in aquantity of SiC also very close to the theoretical quantity without anyexcess of carbon in the carburised foam. In fact, after heat treatmentunder the conditions of Example 1, the weight of foam obtained is 13.30g and, according to (8), than the maximum theoretical quantity is4.17×3.06=12.8 g [sic]. The quantity of residual silicon remains lessthan 1%.

Example 3

A sample of polyurethane foam with a volume of 72 cm³ and an apparentdensity of 0.067 g/cm³ is attacked by a 4% soda solution at 65° C. for 8minutes. A weight loss of 2.33% is noted. The foam, which weighed 4.83 gat the outset, therefore weighs no more than 4.72 g. A mixture of 95%(by weight) furfuryl alcohol and 5% hexamethylenetetramine is preparedas in Example 1. Silicon in powder form with a mean grain diameter of 60μm is added to this mixture in a proportion of 7.9 g of silicon for 10 gof resin. The coefficient b is therefore equal to 0.79. The polyurethanefoam is then immersed in the suspension obtained until the impregnationreaches the heart of the foam. Centrifuging is then carried out so thatthe weight of resin impregnating the sample is reduced to 19.24 g. Theweight of silicon is then 15.2 g and the total weight of the impregnatedsample is 39.16 g. The coefficient a is equal to 19.24/4.71=4.08 and theratio c is 39.16/4.72=8.30, a value greater than:

    c.sub.max =3.917+2.917/b=7.61.

There is therefore an excess of impregnation which will result in aquantity of SiC greater than the theoretical quantity. In fact, afterheat treatment under the conditions of Example 1, the weight of foamobtained is 23.1 g, greater, according to (8), than the maximumtheoretical quantity of 4.17×4.71=19.7 g. The quantity of residualsilicon remains less than 1%.

Example 4

The porous texture of the carbide foam obtained in Example 2 wasdetermined by comparison with that of a carbide foam obtained byreacting carbon foam with silicon monoxide according to the disclosureof the unpublished European patent application 92-420429 and referenced"SiO". Since the latter foam contains, after carburising, an appreciablequantity of carbon which has not reacted, the comparison also relates tothe carbide foams which have undergone a subsequent oxidation treatmentof 3 hours in air at 800° C. intended to eliminate this excess carbon.This treatment was also applied to the foam of Example 2 although itcontains only a little or no free carbon. It is identified in the tablebelow by the comment "oxidised".

    __________________________________________________________________________                                   STANDARD                                                  METH. MAX.ABSC.                                                                            MAX. ORD                                                                             DEVIATION                                      FOAM  PORES                                                                              m.sup.2 /g                                                                          μm  cm.sup.2 /g                                                                          μm                                          __________________________________________________________________________    EX 2  meso nitrogen                                                                            0.0290 0.02106                                                                              0.0139                                                    39                                                                 EX 2                                                                          oxidised                                                                            meso nitrogen                                                                            0.0350 0.01825                                                                              0.0156                                                    26                                                                 SiO   meso nitrogen                                                                            0.0124 0.01825                                                                              0.0156                                                    156                                                                SiO                                                                           oxidised                                                                            meso nitrogen                                                                            0.0124 0.01825                                                                              0.0156                                                    36                                                                 EX 2  meso mercury                                                                             0.0275 0.014  0.0087                                         EX 2  macro                                                                              mercury                                                                             117.5  0.1025 41.97                                          __________________________________________________________________________

The second column indicates the nature of the pores (meso or macropores) revealed by the method used, which is entered in the thirdcolumn: nitrogen adsorption or mercury porosity meter. In the case ofnitrogen adsorption, the BET surface area is also given in this column.The fourth and fifth columns indicate the abscissa (in μm) and theordinate (in cm³ /g) of the maximum value in the distribution of thevolume of the pores as a function of their diameter. Finally the sixthcolumn indicates the standard deviation of the distribution.

The conclusions which can be drawn from this table are as follows:

the carbide foams according to Example 2 of the invention have a bimodaldistribution whether or not they have been oxidised: a first mode is at0.0290 μm (non-oxidised foam) or at 0.0350 μm (oxidised foam) withrespective standard deviations of 0.139 and 0.156 μm. The effect of theoxidation treatment is to increase the mean diameter of the mesopores;

the carbide foams obtained from gaseous SiO have a distribution ofmesopores centred on an adjacent value which is lower before oxidationand of the same order after oxidation;

the carbide foams according to Example 2 have a second mode at 117.5 μm(macropores). In FIG. 1, the distribution of the pores of the sample ofExample 2 is shown. It can be seen very clearly that the distribution ofthese macropores forms a very sharp peak centred on 100-150 μm. This isone of the essential characteristics of these foams.

finally, good agreement is found between the distributions of porescalculated by adsorption-desorption of nitrogen and those measured withthe mercury porosity meter.

Example 5

All the following examples were carried out by impregnating apolyurethane foam with a weight of 1.05 to 1.14 g and a porosity of 20pores per inch (25.4 mm); the impregnation mixture consists of furfurylresin (130 g), hexamethylenetetramine (5% by weight of the resin) andsilicon with a particle size of 3 to 5 microns under the conditionsgiven below. The specific surface areas are measured by adsorption ofnitrogen after pyrolysis of the product in air (SiC without any residualcarbon).

1. Effect of the thermal profile and of the incremental temperature:

    ______________________________________                                                                      linear                                                                        velocity                                                                      of argon                                                                              specific                                                 thermal      in the  surface                                 b        c       profile      reactor area                                    ______________________________________                                        Test 1                                                                              0.7    7.8     A*         0.4 cm/s                                                                              7.8 m.sup.2 /g                        Test 2                                                                              0.7    7.5     B**        0.4 cm/s                                                                              4.7 m.sup.2 /g                        Test 3                                                                              0.7    7.6     C***       0.4 cm/s                                                                              1.3 m.sup.2 /g                        where    A*:     5° C./min                                                                            20° C. → 250° C.                           1° C./min                                                                            250° C. → 250° C.                          3° C./min                                                                           1000° C. → 1350° C.                         3 h at 1350° C.                                                B**:    1° C./min                                                                           20° C. → 800° C.                            0.3° C./min                                                                         800° C. → 1350° C.                          3 h at 1350° C.                                                C***    5° C./min                                                                            20° C. → 250° C.                           1° C./min                                                                            250° C. → 1000° C.                         3° C./min                                                                           1000° C. → 1500° C.                         3 hr at 1500° C.                                      ______________________________________                                    

2. Effect of the flow rate of inert gas (or linear velocity of the inertgas in order to get away from the geometry of the reactor)

    ______________________________________                                                                      linear specific                                                    thermal    velocity                                                                             surface                                  b           c      profile    of argon                                                                             area                                     ______________________________________                                        Test 1  0.7     7.8    A*        0.4 cm/s                                                                             7.8 m.sup.2 /g                        Test 4  0.7     7.5    A*        0.2 cm/s                                                                            14.4 m.sup.2 /g                        Test 5  0.7     7.5    A*       0.08 cm/s                                                                            20.4 m.sup.2 /g                        ______________________________________                                    

3. Effect of factor b

    ______________________________________                                                                      linear specific                                                    thermal    velocity                                                                             surface                                  b           c      profile    of argon                                                                             area                                     ______________________________________                                        Test 1  0.7     7.8    A        0.4 cm/s                                                                              7.8 m.sup.2 /g                        Test 6  0.5     7.5    A        0.4 cm/s                                                                             13.2 m.sup.2 /g                        ______________________________________                                    

We claim:
 1. Silicon carbide for use as a catalyst or catalyst carrierand having a large open macroporosity for access for reaction gases,said silicon carbide characterized by:a) a BET surface area between 10and 50 m² g; b) a bimodal porosity comprising a mesoporosity centered ona pore diameter between 0.0275 and 0.0350 μm with a standard deviationof less than 0.0160 μm, and a macroporosity centered on a pore diameterbetween 100 and 150 μm with a standard deviation less than 50 μm; and c)a residual silicon content of not more than 0.1% by weight.
 2. Siliconcarbide according to claim 1, having a coefficient k₁ of darcianpermeability between 4 and 12×10⁻⁹ m², and a coefficient k₂ ofnon-darcian permeability between 1.1 and 1.7×10⁻⁴ m².
 3. Silicon carbidefor use in filtering diesel engine exhaust gases, characterized by:a) aBET surface area less than 5 m² /g; b) a porosity comprising a verysmall mesoporosity and a macroporosity centered on a pore diameterbetween 100 and 150 μm with a standard deviation between 30 and 50 μm;and c) a residual silicon content of no more than 0.1% by weight.
 4. Acatalyst or catalyst support comprising silicon carbide according toclaim
 1. 5. A filter for diesel exhaust gases comprising silicon carbideaccording to claim 3.